In the course of the chemical modification of certain cephalosporin antibiotics it is often convenient to protect the reactive cephalosporin C.sub.4 carboxylic acid function as a p-nitrobenzyl ester. The ester protecting group renders the carboxylic acid function inert to the various chemical operations performed on other parts of the cephalosporin molecule. Once the chemical operations have been accomplished, the p-nitrobenzyl group can be removed to provide the modified antibiotically active cephalosporin carboxylic acid.
Various chemical methods of removing ("deblocking") the p-nitrobenzyl group have been described in the cephalosporin literature. Such treatment involves a rigorous acid hydrolysis or, in some instances, hydrogenolysis. The unsubstituted benzyl group itself is cleaved by base and also by strongly acidic reagents, although the extent of hydrolysis with acids is variable and is dependent upon the concentration of acid and upon other conditions such as temperature and time. The effect of the electron attracting p-nitro substituent on the benzyl group is to stabilize the resulting p-nitrobenzyl ester towards acids. Thus p-nitrobenzyl esters display increased resistance to acid hydrolysis, hence more rigorous hydrolysis conditions are required. The particular attraction of this class of ester protecting groups for cephalosporin chemists lies undoubtedly in their ready removal by hydrogenolysis. For a resume of the methods of removal of benzyl ester protecting groups see J. F. W. McOmie, Protective Groups in Organic Chemistry,Plenum Press, London and New York, 1973, Chapter 5. The use of acid deblocking methods with certain cephalosporin p-nitrobenzyl esters, because of the rigorous acidic conditions, often produces polymeric impurities which make separation and purification of the final product difficult.
The p-nitrobenzyl cephalosporin esters (PNB esters) which are required as starting materials in the process of this invention can be prepared by the ring expansion of the corresponding penicillin sulfoxide PNB esters. The starting materials can be described, conveniently, by the structural formula ##SPC1##
Wherein R is the residue of the acyl group, for example, one derived from the penicillin sulfoxide PNB ester used. These starting materials can be prepared by the thermal rearrangement of the corresponding penicillin sulfoxide PNB esters as described in U.S. Pat. Nos. 3,275,626, 3,632,850 and 3,781,283. Numerous penicillins derived by fermentation methods known in the prior art (U.S. Pat. Nos. 2,379,295 and 2,479,297; 2,562,407 to 2,562,411; and 2,623,876) can be converted to PNB esters sulfoxides and rearranged to 7-acylamidocephem PNB esters. Alternatively the PNB ester starting materials can be prepared by esterifying the corresponding cephem-4-carboxylic acid intermediate with p-nitrobenzyl bromide in the presence of base and recovering the cephem-4-carboxylate PNB ester as described throughout the cephalosporin art.
Two highly useful cephalosporin intermediates are the p-nitrobenzyl esters of 7-(D-.alpha.-aminophenylacetamido)-3-methyl-3-cephem-4-carboxylic acid and 7-amino-3-methyl-3-cephem-4-carboxylic acid (7-ADCA). The former is significant because, upon cleavage of the p-nitrobenzyl group, the well recognized antibiotic, cephalexin, results; therefore, the p-nitrobenzyl ester finds great use as an intermediate to the preparation of cephalexin. The latter, the p-nitrobenzyl ester of 7-ADCA, is a useful intermediate in the preparation of active cephalosporin antibiotics. All the cephalosporin acid products derived by the process of this invention are known antibiotic substances described in the cephalosporin art.
Heretofore enzymatic methods for the removal of the p-nitrobenzyl ester protecting group from cephalosporin compounds have not been described. It is an object of this invention to provide an alternative process for the removal of the p-nitrobenzyl group from cephalosporin esters and particularly those which may be sensitive to chemical deblocking, particularly by acids.